In a major TV (television) broadcast system, for example, the NTSC (National Television System Committee) TV broadcast system, 60 field images of 262.5 horizontal scanning lines are displayed each second to equivalently display 30 frames of images (the number of horizontal scanning lines per frame=525 lines) each second. Such a system combining two field images to make and display a frame image is called an interlaced system. On the contrary, a system displaying a frame image at a time without dividing the number of scanning lines is called a progressive system. While, for example, a field image of 262.5 horizontal scanning lines is displayed every 1/60 of a second in the interlaced system, in the progressive system, a frame image of 525 horizontal scanning lines is displayed every 1/60 of a second, so screen flicker is reduced, thereby a high-resolution image can be displayed. In some cases, a video standard in which a frame image of 525 scanning lines is displayed by the interlaced system is called “525i”, and a video standard in which a frame image of 525 scanning lines is displayed by the progressive system is called “525p”.
Conventionally, an IP converter for converting an interlaced image into a progressive image is known. When an interlaced image (for example, a field image of 262.5 scanning lines) is converted into a progressive image (for example, a frame image of 525 scanning lines), it is required to interpolate pixel data for a part which is not included in the original field image.
In the IP conversion, a process of interpolating pixel data changes depending upon a signal source in general. More specifically, in the case of still pictures, inter-field interpolation is performed, and in the case of moving pictures, intra-field interpolation is performed. For example, as shown in FIG. 11, assuming that a pixel h in a field FE2 is an interpolation point, in the inter-field interpolation, data is interpolated in the field FE2 to be interpolated using data in previous and following fields FE1 and FE3. On the other hand, in the intra-field interpolation, data is interpolated using data in the field FE2 to be interpolated. In the fields FE1, FE2 and FE3 in FIG. 11, a solid line indicates a line where video data actually exists, and a broken line indicates a line where interpolation data is produced.
Referring to FIG. 12, the concept of typical intra-field interpolation will be described below. FIG. 12 shows part of an arbitrary field image in the interlaced system. In the drawing, lines W and X indicate pixel lines (actual data lines) where video data actually exists. A line Y indicates a line where video data exists in a field previous to or following the present field as well as a line (interpolation data line) where interpolation data is produced during intra-field interpolation. A symbol ∘ on each line indicates a position where a pixel exists.
In FIG. 12, assuming that the pixel h is a point to be interpolated, as a simple technique of intra-field interpolation, for example, there is a method using a value (average value) determined by dividing the sum of pixel data (such as luminance or chroma data) of pixels c and m directly above and directly below the pixel h by two as interpolation data. As another technique of intra-field interpolation, there is a technique called “diagonal interpolation”. A method of interpolating data in a pixel to be interpolated from two adjacent actual data lines W and X above and below the pixel by the diagonal interpolation is described in, for example, Published Japanese translation of PCT International Publication for Japanese Patent Application No. 2001-506113.
The diagonal interpolation is a technique of determining interpolation data by calculation referring to not only pixels directly above and directly below the pixel to be interpolated but also pixels in diagonal directions. For example, interpolation data is determined by calculation referring to pixels on the actual data lines W and X on not only an interpolation axis C1 in a vertical (central) direction but also interpolation axes L2 and L1 in left diagonal directions and interpolation axes R2 and R1 in right diagonal directions around the pixel h which is the interpolation point. In this case, a combination with the strongest correlation is detected among combinations of pixels on each interpolation axis, that is, (a−o), (b−n), (c−m), (d−l) and (e−k), and by using the data, data of the pixel h is interpolated.
In order to detect correlation, for example, subtraction of each pixel data is performed to obtain its absolute value. For example, the correlation of the combination of the pixels (d−l) is expressed by Formula 1 below. ABS means obtaining an absolute value. In this method, in the case of data with a strong correlation, the value of Formula 1 is small.ABS(d−l)  (Formula 1)
However, in the method using Formula 1, only correlation between one pixel and one pixel is determined, so even if data is originally and completely irrelevant to a video sequence, a strong correlation may be shown (that is, a wrong interpolation axis may be selected), thereby inadequate interpolation data is often produced. For example, as shown in FIG. 13, in the case of an image with the shape of a thin line in a direction of the interpolation axis R1, an interpolation axis in a direction different from an adequate direction may be selected by mistake. Therefore, in general, correlation is detected using some data considered as a group.
For example, as shown in Formula 2 below, in order to detect correlation between data of a pixel d and data of a pixel 1, a data group 2-1 (c, d and e) around the pixel d and a data group 2-2 (k, l and m) around the pixel l are used, thereby errors in correlation detection can be reduced.ABS(c−k)+ABS(d−l)+ABS(e−m)  (Formula 2)
In the case of Formula 2, like Formula 1, the stronger the correlation is, the smaller the calculation result becomes. However, even in a method using Formula 2, in the case of an image with a strong correlation in right and left diagonal directions, a strong correlation in a wrong diagonal direction is detected, thereby as a result, inadequate interpolation data may be produced.
Thus, in the intra-field interpolation using conventional diagonal interpolation, in the case where an optimum reference pixel is detected from two adjacent pixel lines above and below the interpolation point in a diagonal direction, when an pixel with the same luminance exists, a strong correlation is shown, thereby resulting in a problem that a pixel in a diagonal direction irrelevant to a video sequence is interpolated by mistake.
In view of the foregoing, it is an object of the invention to provide an image processing apparatus, an image processing method a video display apparatus and a recorded information reproducing apparatus capable of reducing errors in interpolation in the case of using diagonal interpolation to perform high-quality intra-field interpolation.